This invention relates to improvements in methods and apparatus for the continuous removal of a sub-quantity representative test quantity from a gas stream and for the continuous mixing of the test quantity with a dilutant gas stream. The sub-quantity is removed and branched off from a main gas flow stream such as the exhaust stream from a diesel engine. A counter-pressure is employed to act on the sub-quantity from the dilutant stream to control the volume stream of the sub-quantity. The invention also relates to an improved apparatus for utilizing the method of the invention including a probe line which branches off of the sub-quantity discharge line and a mixing tunnel in which the downstream end of the sub-quantity removal and a delivery line carrying the dilutant stream discharge for mixing the gases.
The method and apparatus of the invention are particularly well suited for measuring processes to determine characteristics of a gas stream where only a representative sub-quantity may be examined to avoid any material disruption of the main gas stream and to accommodate required dimensioning of measuring instruments. Also, applications utilizing the method are possible which serve to produce gas mixtures for a variety of other uses. For example, analysis of constituents in suspended material in gases from chemical processes may be accomplished. One particular use is the measurement of exhaust gases from internal combustion engines and particularly diesel engines. In the case of measuring the exhaust gases, it is frequently necessary to dilute the gas stream to be analyzed with other gaseous dilutant streams such as air. In such employment, dilutant tunnels are used where a test sample is fed for measuring the particle emission of the diesel engines. The exhaust gas of the engine is introduced into the dilutant tunnel and is uniformly mixed with pure air in a turbulent flow in order to simulate the emission conditions. It is frequently desirable for space and performance reasons to extract only a sub-quantity from the gas stream and to examine this after dilution. A critical problem in such gas stream dilution is the control of the sub-quantity removed since it is usually not appropriate to directly control the stream of the sub-quantity with throttle valves or similar mechanisms, and the influence or variation of the composition of the sub-quantity by its removal must be avoided under all circumstances to obtain a proper test mount. In a method of testing and diluting the substream of diesel exhaust gas, for example, the pressure in the diluting system or dilutant stream has been varied by respective blowers arranged at the input side and output side of the diluting system. As a result of this, the counter-pressure acting on the sub-quantity from the dilutant stream at the downstream side is directly influenced, and thus the volume of the sub-quantity is affected or controlled. This method heretofore used and the apparatus used for this has critical disadvantages.
The temperature of the dilutant stream being mixed with the sub-quantity rises as a consequence of the pressure elevation of the dilutant stream at the input side as well as a result of compressor losses that occur. This has an influence on the measurement of the sub-stream. To avoid this, this requires a relatively involved cooling of the dilutant stream. In the particle measurement of diesel exhaust gases, for example, an admission temperature of the dilutant air of 25.+-.5.degree. C. as well as a maximum temperature of 52.degree. C. of the gas sample ultimately examined are prescribed for satisfactory results.
As a result of utilizing the foregoing known principles, an exhaust arrangement acting on the diluting stream at the output side must be controlled with a full throughput quantity of the dilutant stream which requires a rather high powered drive and correspondingly a high outlay for the control is thus required. Another critical disadvantage of the method and arrangement heretofore known results in that there is a maximum counter-pressure that can operate in the exhaust gas flow system of a diesel motor. This is important because a relatively high gas counter-pressure (such as up to about 250 mbar) can appear in certain operating points with the us of various exhaust system configurations, and specifically with soot filters. When it is desired to control the pressure in the actual diluting system via the exhaust mechanism at the output side, then in addition to causing increase in the drive power of the blower, this results in a further temperature elevation of the dilutant air and further aggravates the cooling problems.